This invention relates to a low adhesion backsize and release coatings of the type employed in connection with normally tacky and pressure-sensitive adhesive coated articles and more particularly to self-associating low adhesive backsize coatings.
Normally tacky and pressure-sensitive adhesive (PSA) materials have been used for well over half a century. Products of this type, which take the form of tapes, labels, and other types of adhesive coated articles, must be protected from unintended adhesion to other surfaces. Hence, tapes are typically wound into a roll on their own backing and labels and sheets are typically laminated to a release sheet or one on top of another to prevent their accidental adhesion to other surfaces and also to prevent their contamination with air-borne dust and other contaminants. In order to allow the roll to be unwound without the undesirable transfer of adhesive to the tape backing, or to permit an adhesive-coated sheet to be removed from a stack of similarly coated sheets, it is customary to provide the tape backing or the surface of a successive sheet with a low adhesion backsize (LAB). Similarly, the release sheet or liner to which the adhesive coated article is typically laminated is supplied with a release coating to permit the easy removal of the liner from the coated article.
This LAB or release coating is expected to reproducibly provide an appropriate level of release from the adhesive of interest, to not deleteriously affect the adhesive, and to be resistant to aging so that the release level remains relatively stable with time. In recent years, as competition has expanded in the PSA industry, a need to differentiate product performance as well as more demanding product requirements has led to recognition of the importance of release level. D. Satas, Chapt. 23 xe2x80x9cRelease Coatingsxe2x80x9d, Handbook of Pressure Sensitive Adhesive Technology, Second Edition, D. Satas, ed., Van Nostrand Reinhold, 1989, defines seven distinct levels of release, ranging from xe2x80x9csuper low releasexe2x80x9d (0.15-0.30 N/dm) to xe2x80x9cvery tight releasexe2x80x9d (20-80 N/dm). In many circumstances it is important for the LAB to possess other properties besides functioning as a release agent. For example, the release coating on masking tape must possess good solvent resistance in addition to providing a surface to which paint can adhere.
Release coatings and LABs are typically applied to substrates at coating weights around 1 g/m2. In order to obtain such thin coatings, dilute solutions (2 to 5% solids) of the coating compositions in organic solvents have traditionally been used. Recent efforts have been directed to delivering such coatings at high or 100% solids or from aqueous media, thus reducing the environmentally damaging hydrocarbon emissions, conserving precious natural resources, and lowering economic cost. Despite the numerous efforts to produce release coatings at high or 100% solids or from aqueous media, such coatings have still required a complex combination of components and involved polymerization processes in order to insure adequate release levels, as well as minimizing release coating transfer.
For example, a release coating is described comprising a hydrocolloid stabilized aqueous emulsion of a crosslinkable film forming vinyl polymer having a glass transition temperature of from 0xc2x0 to 60xc2x0 C. and from 0.5 to 20% by weight on emulsion polymer solids of a release promoting compound including silicone copolyols, hydrocarbon or fluorocarbon Werner complexes, or organofunctional siloxanes. Unfortunately, this coating requires cross-linking.
Another example discloses a substrate coated with a release coating composition consisting essentially of polyvinyl alcohol, a migratable release-promoting agent, a water-soluble salt of a coordinating metal, and a water soluble boron compound. The release-promoting agent is functionally defined as a surfactant that gives a release value less than about 162.5 grams per centimeter for masking tape when formulated at 5% loading into polyvinyl alcohol. Not only does this example require cross-linking, but uses salts of a coordinating metal, which can be toxic.
One conventional method improves the releasability of a backing layer by applying a mixture comprising a polymer, preferably having acid functionality, and an aqueous solution or dispersion of an organic compound having at least one fatty acid ester and at least one quaternary amine, such as lecithin. However, the use of an ionic interaction to develop anchorage to a substrate lends itself to moisture sensitivity.
Yet another aqueous release coating for pressure sensitive adhesive tapes attempts to provide a release coating comprising a latex film forming polymeric material, a release promoting additive, and a microdispersed polyamide resin having a softening point greater than 70xc2x0 C. As described, preferred release promoting additives include N-alkyl sulfosuccinamates, alkyl sulfosuccinates, alkylaryl polyalkylene oxides, salts of long chain alkyl sulfates, and amine polyglycol condensates. It has been observed that initially there is transfer of the release coating to the adhesive and such observation is supported by heat aging data that reveals loss of tack (using a rolling ball test).
Another example of an aqueous based release coating composition suitable for a repositionable adhesive that comprises about 10 to about 80 parts by wet weight of a sulfosuccinamate surfactant and about 20 to about 90 parts by wet weight of an acrylic copolymer. These compositions appear to rely solely on ionic interactions, which tend to be sensitive to humidity.
Therefore, what is needed is an aqueous release coating composition that can be prepared by blending commercially available materials, hence does not require a polymerization process; that can provide a wide range of release levels for both permanent and repositionable pressure sensitive adhesives with the level of release determined by the amount and nature of the components; that does not require post-coating crosslinking reactions to provide stable release performance and minimal loss in readhesion values; and that is not sensitive to humid environments. A further need is a release coating prepared from water based materials that does not require organic cosolvents to provide acceptable release performance on coating and drying. A still further need is a release coating delivered from water with the above attributes that is capable of being written on with water based pens and which securely anchors paint and ink.
Briefly, in one aspect of the present invention, a low adhesion backsize coating is provided comprising:
(1) 25% to 75% by weight on a solids basis of a functional polymer wherein the functional polymer has
(a) a functional moiety,
(b) is non-tacky at room temperature,
(c) is water dispersible or water-based or a latex emulsion,
(d) optionally, may be a copolymer,
(e) optionally, may be crosslinked and
(2) 75% to 25% by weight on a solids basis of a release-promoting additive having a molecular weight of 10,000 or less, wherein the release-promoting additive is
(a) a bifunctional material, wherein such bifunctional material has
(i) a release moiety at one end, and
(ii) an anchoring moiety at the other end has a complementary function to the functional moiety of the functional polymer, and
(b) is water dispersible or water miscible.
The functional polymer in combination with the release-promoting additive advantageously minimizes surface energy and cohesive failure. The functionality of the functional polymer is selected in such way as to promote ionic interactions, acid-base interactions, and/or hydrogen bonding. It is preferable that the combination of the functional polymer and the release promoting additive work in such a fashion as to provide more than one interaction per molecule of each component, for example multiple hydrogen bonds or acid-base interactions. Although some covalent bonding may be tolerated between the functional polymer and the release promoting additive, it is preferred that there be no covalent bonding, but rather the weaker interactions such as described above.
This invention describes a low adhesion backsize (LAB) material made by mixing together materials, which will self-associate. The mixtures can be waterbased and have high solids levels. The release is provided by functional groups such as silicones, fluorocarbons or long chain hydrocarbons. Release level can be tailored with release levels as low as the 0.5 gram/cm range to microsphere adhesives.
By using commercially available materials and not having to conduct a polymerization process the cost of the LAB is the cost of the components. The samples made thus far have exhibited not only low release levels but also high writeability and high lithographic print density. The current invention couples low to moderate molecular weight surface active materials (silicones, fluorocarbons, long chain hydrocarbons) which also possess a moiety available for interaction with a film forming polymer via ionic, acid-base, or hydrogen bonding providing good anchorage. During the drying process, it is believed that the low molecular weight of the surface-active material allows it to migrate readily to the interface prior to anchoring.
Alternately the two components can be coated in layers instead of mixing together and coating. For example the functional base polymer can be coated and dried on the substrate, followed by coating and drying of the release promoting additive. Advantageously, one of the coatings can be applied to the substrate and subsequently, the second layer could be applied. For example, the functional polymer layer could be applied at the time a substrate is manufactured. When used, this substrate could then be coated with the release promoting additive material, thus providing the combination of the present invention. This would be advantageous, for example, when the functional base polymer is included in a coating that is needed on the substrate, such as a matte coating on a film substrate.
Functional Base Polymer
Useful functional base polymers are those that are water dispersible, water-based, or a latex emulsion material. General physical properties include glass transition temperatures above room temperature and film formation upon drying. In addition, the polymer may be a copolymer and may be crosslinked. From a chemical nature, the polymer unit includes functional moieties that complement the functional groups on the release component. Such functionalities include hydrogen bond donors, such as carboxylic acids or alcohols; hydrogen bond acceptors, such as substituted amides or ethers; acidic groups; basic groups; anionic groups; cationic groups or combinations thereof. Particularly useful types of such polymers containing these functionalities include but are not limited to (meth)acrylates, vinyl acetates, styrenes, polyurethanes, cellulosics, and the like. Preferred polymers are the (meth)acrylates. Most preferred are carboxylic (meth)acrylates, which are commercially available. Specific examples include Rohm and Haas Acusol or Acrysol series of carboxylic latices and B. F. Goodrich Carboset series.
Release Promoting Additive
The release promoting additive materials useful in this invention are bifunctional. By xe2x80x9cbifunctionalxe2x80x9d it is meant that the release promoting additive material has at least two portions wherein one portion has a release moiety and the other portion an anchoring moiety. Useful materials have a molecular weight generally less than 10,000, preferably less than 5,000 since it is believed that lower molecular weight allows for mobility of the material to the air interface during drying. Also, the additive should be water dispersible or water miscible. Further, the release moiety can be a long chain hydrocarbon, a fluorochemical, or a silicone. Preferably, the poly(dimethyl siloxane) group is used as it provides low release with minimal loading.
The anchoring moieties include hydrogen bond donors, such as carboxylic acids or alcohols; hydrogen bond acceptors, such as substituted amides, or ethers; acidic groups; basic groups; anionic groups; or cationic groups. This anchoring group is chosen to provide a complementary functionality to that of the functional base polymer.
Examples of such release promoting additive materials having bifunctionality include nonionic surfactants, long chain alkyl carboxylic acids and their salts; silicone copolyols and fluorochemical surfactants. Examples of commercially available water dispersible or water soluble materials include, but are not limited to: Brij and Tween surfactants available from ICI Chemicals; Silwet silicone surfactants from Witco Corp.; the silicone surfactant series from Dow Corning; and the Zonyl series of fluorochemical surfactants from Du Pont.
Combinations of Functional Base Polymer and Release Promoting Additive
The combination of the functional base polymer and a release promoting additive are chosen such that the anchoring moiety of the release promoting additive and the functional moiety of the functional base polymer are complementary. This is to provide interactions such as hydrogen bonding, acid-base interaction, or ionic interaction. It is preferable that the combination of the functional polymer and the release promoting additive work in such a fashion as to provide more than one interaction per molecule. For example, a polyether chain with a polyacrylic acid chain form multiple hydrogen bonds between the two chains.
By choosing the components properly, the resulting release surface will have minimum surface energy, and good cohesive strength. To maintain this balance, the functional polymer is generally in the range from 25 to 75% by weight of the composition and a release promoting additive is 75 to 25% by weight of the composition.
The low adhesion backsize coating of the present invention can be a combination of more than one functional polymer with a single release promoting additive, a single functional polymer with more than one release promoting additive and more than one functional polymer with more than one release promoting additive. Furthermore, each functional polymer and each release promoting additive may have more than one functional moiety.
Adhesives
Release coatings of this invention can generally be used with a variety of pressure sensitive adhesives. Types of adhesives include but are not limited to acrylics, tackified rubber resins, and tackified block copolymers ranging in adhesive strength from permanent to removable.
Substrates
The release coatings defined above are best used as a coating for a solid substrate, which may be a sheet, fiber or shaped object. However, the preferred substrates are fixed flexible substrates such as are used for pressure sensitive adhesive products. Suitable substrates include paper, coated paper such as polymeric coated or saturated paper (for example polyethylene coated kraft paper), metal sheets and foils, non-woven fabrics, and films of thermoplastic resins such as polyesters, polyamides, polyolefins, polycarbonate, polyvinyl chloride, etc., although any surface requiring release toward adhesives can be used. Primers known in the art can be utilized to aid in adhesion of the coating to the substrate, but they are not generally necessary.
Coating Methods
Release coating compositions of the present invention may be applied to suitable substrates by means of conventional coating techniques such as wire-wound rod, direct gravure, offset gravure, reverse roll, air-knife, and trailing blade coating; hot melt coating is also possible.
The release coatings and the LABs of the coated sheet material are typically applied at about 0.2 to about 2.0 grams/m2 depending upon the nature of the flexible substrate and the intended application of the coated sheet material.
Articles
Articles that may be prepared using the release coating compositions of the present invention include but are not limited to a roll of tape, a tape with a release liner and a transfer tape. A roll of tape comprises a flexible backing member, a pressure sensitive adhesive coating on one major surface of the backing member and a release coating on the opposite major surface of the backing comprising the low adhesion backsize defined above. Furthermore, the tape with a release liner comprises a flexible backing member, a pressure-sensitive adhesive coating on one major surface of the backing member and a release liner comprising a flexible sheet coated over the major surface adhered to the pressure-sensitive coating with the low adhesion backsize defined above. The transfer tape comprises a film of pressure-sensitive adhesive between two release liners, at least one being coated with the low adhesion backsize of this invention.
Additional articles include but are not limited to a single coated sheet material, a stack of coated sheets, a fan-folded web of coated sheet material and a roll of coated sheet material. Such coated sheet materials generally comprise a coating of the release coating composition of the present invention on at least a portion of one side of the sheet and an adhesive is on at least a portion of the other side. Typically, the adhesive is a normally tacky and pressure-sensitive adhesive. The stack of superimposed sheets of the coated sheet material comprises a series of pressure-sensitive adhesive coated sheets being in contact with a release coating composition portion of an immediately adjacent sheet.
Alternatively, the stack of superimposed sheets can be configured to be a fan folded web formed from the coated sheet material, wherein an adhesive coating is on each segment of the web being in contact with a low adhesion backsize on an immediately adjacent segment.
Additional articles include, but are not limited to:
(a) a coated sheet material having a low adhesion backsize on one side and adhesive on the other side wherein said coated sheet material could be wound convolutedly on itself about a core to form a roll;
(b) a coated sheet material wherein the low adhesion backsize covers the first portion of one side and any normally tacky pressure- sensitive adhesive covers a second portion of the same side;
(c) a coated sheet material wherein the sheet is an elongated strip having spaced alternately areas of low adhesion backsize and adhesive; and
(d) a coated sheet material wherein the sheet is generally rectangular, the low adhesion backsize being present in a band adjacent one edge and pressure-sensitive adhesive being present in a band adjacent the opposite edge.
Alternately the two components can be coated in layers instead of mix and coating. For example the functional base polymer can be coated and dried on the substrate, followed by coating and drying of the release promoting additive. This would be advantageous, for example, when the functional base polymer is included in a coating that is needed on the substrate, such as a matte coating on a film substrate.
This invention is further illustrated by the following examples that are not intended to limit the scope of the invention. In the examples, all parts, ratios and percentages are by weight unless otherwise indicated. The following test methods were used to evaluate and characterize the self-associating low adhesion backsize produced in the examples.